Allergic Diseases also known as Allergies, is an abnormal reaction of the body to a previously encountered allergen introduced by inhalation, ingestion, injection, or skin contact, often manifested by itchy eyes, runny nose, wheezing, skin rash, or diarrhea.

Allergies can develop at any age; the risk of developing allergies is genetic. It is related to ones family history of allergy. If neither parent is allergic, the chance for allergies is about 15%. If one parent is allergic, the risk increases to 30% and if both are allergic, the risk is greater than 60%.

Common allergens include pollen and certain food. Metals and other substances may also cause problems. Food, insect stings, and medications are common causes of severe reactions. Their development is due to both genetic and environmental factors.

Allergies are common. In the developed world, about 20% of people are affected by allergic rhinitis, about 6% of people have at least one food allergy, and about 20% have atopic dermatitis at some point in time. Depending on the country about 1–18% of people have asthma. Anaphylaxis occurs in between 0.05–2% of people. Rates of many allergic diseases appear to be increasing.

Causes of Allergies

An allergy starts when our immune system mistakes a normally harmless substance for a dangerous invader. The immune system then produces antibodies that remain on the alert for that particular allergen. When we are exposed to the allergen again, these antibodies can release a number of immune system chemicals, such as histamine, that cause allergy symptoms.

Latex or other substances you touch, which can cause allergic skin reactions

Symptoms of Allergies

Allergies can be seasonal, worsening during certain times of the year, such as pollen allergies in the spring or perennial (occurring year-round). Common allergens include weed or grass pollen, dust mites, animal dander, mold, insect stings and a variety of food types, such as eggs, shellfish, nuts and grains. Allergic reactions can range from mild to severe. In some severe cases, allergies can trigger a life-threatening reaction known as anaphylaxis.

Hay fever, also called allergic rhinitis, may cause:

Sneezing

Itching of the nose, eyes or roof of the mouth

Runny, stuffy nose

Watery, red or swollen eyes (conjunctivitis)

A food allergy may cause:

Tingling mouth

Swelling of the lips, tongue, face or throat

Hives

Anaphylaxis

An insect sting allergy may cause:

A large area of swelling (edema) at the sting site

Itching or hives all over the body

Cough, chest tightness, wheezing or shortness of breath

Anaphylaxis

A drug allergy may cause:

Hives

Itchy skin

Rash

Facial swelling

Wheezing

Anaphylaxis

Atopic dermatitis, an allergic skin condition also called eczema, may cause skin to:

Itch

Redden

Flake or peel

International differences have been associated with the number of individuals within a population have allergy. Allergic diseases are more common in industrialized countries than in countries that are more traditional or agricultural, and there is a higher rate of allergic disease in urban populations versus rural populations, although these differences are becoming less defined.